The present invention relates to a device for determining the direction of flow of a fluid of substantially directed flow, with known mass flow, having a flow straightener and a sensor element associated therewith which gives off electric signals.
The fuel injection of internal combustion engines is controlled as a function of the mass flow of the intake air. For the measurement of the mass flow, use is made in many cases of a mass-flow measuring device which operates in accordance with the eletrothermal principle. Such a measuring device is provided, for compensation of the temperature, with at least one temperature-measuring element and the mass-flow measuring device proper, the main component of which is an electrically heatable resistance element which is arranged in the mass flow. The heat removed from this heating element is a measure of the mass flow, more heat being removed the stronger the mass flow is.
Such a mass-flow measuring device is very insensitive to the direction of the flow since, regardless of whether the flow is directed forward or backward, practically the same electric signal is given off, corresponding to the heat removed. In other words, with a known mass-flow measuring device one can merely determine the amount of the mass flow, but not its direction. It has now been found that in the intake system of internal combustion engines oscillations of the air column occur under certain operating conditions, they being superimposed on the constant flow so that a rapidly pulsating flow results therefrom. Depending on the rpm and the number of cylinders, the natural frequency of the oscillation is within the range of between 10 and 1000 Hz. Depending on the velocity of flow and the amplitude of oscillation, reverse flows occur for short periods of time. As a result of the principle of measurement of the mass-flow measuring device described above, reverse flows produce the same measurement signal as intake flows, so that in the final analysis too large an amount of fuel is fed to the engine.
Direction-sensitive flowmeters are known which have two resistance elements which are arranged on a common support and are installed one behind the other in the direction of flow. Such flowmeters, however, are not suitable, in view of their inertia, for the detecting of rapid changes in flow.